This patch implements a timezone backend which is based on ICU, rather
than operating system calls. It can be turned on by passing the
--icu-timezone-data flag. The goal here is to take advantage of ICU's
data, which is more complete than the data that some system calls expose.
For example, without any special code, this patch fixes the time zone
of Lord Howe Island to have a correct 30 minute DST offset, rather than
60 minutes as the OS backends assume it to have.

Unfortunately, the parenthized timezone name in Date.prototype.toString()
differs across platforms. This patch chooses the long timezone name,
which matches Windows behavior and might be the most intelligible, but
the web compatibility impact is unclear.

BUG=v8:6031,v8:2137,v8:6076

Review-Url: https://codereview.chromium.org/2724373002
Cr-Commit-Position: refs/heads/master@{#44562}